The objective of this study is to characterize the distribution of activity of drug metabolizing enzymes measured in the Pittsburgh 5 drug cocktail as phenotypic traits in the general population (both in Afro-Americans and Caucasians) over a wide age range (18-80 years) and to compare the activities of these enzymes with known genotypic traits. The drugs caffeine, mephenytoin, debrisoquine, chlorzoxazone and dapsone are being used as selected probes for CYP1A2, 2C19, 2D6, 2E1 and 3A4 respectively.